84 Prof. Draper on the Phosphorescence of Bodies. 



stouCj Baldwin's compound and the diamond, in reality all solid 

 substances, are phosphorescent when properly treated, except 

 perhaps the metals ; and even these he believed would eventually 

 be found to have the same property. 



There is one point on which Dufay dwells which deserves more 

 than a passing remark, — the connexion between phosphorescence 

 and temperature. He proved that phosphori cannot absorb light 

 so well when they are warm as when they are cold, and that a 

 rise of temperature always makes them disengage their light. 



It is obvious that these early investigators laboured under 

 great difficulties, arising from the imperfect chemical science of 

 those times. They confounded together things that were essen- 

 tially distinct, such as the shining of urine-phosphorus with the 

 glow of the diamond ; and this again with the electrical light 

 arising when friction has been employed. Then, again, their 

 erroneous views of the composition of bodies were constantly lead- 

 ing them astray. Thus Dufay, finding that the Bolognian stone 

 (sulphuret of barium) emitted a sulphury smell, and thinking 

 that it shone because of the burning of the sulphur, transferred 

 the same kind of I'easoning to the case of yellow diamonds, and 

 asserted that they also glowed through the combustion of the 

 sulphur that discoloured them. 



I do not intend now to give a review of the subsequent disco- 

 veries and hypotheses brought forward by the numerous experi- 

 menters in the last century, or by Heinrich, the Becquerels, Biot, 

 Poggendorff, Pearsall, and many others in this. I may, however, 

 recall attention to a very elaborate memoir by Osann (Poggen- 

 dorff's Annalen, vol. xxxiii. p. 405. 1834), in which he discusses 

 the various theories of combustion, absorption, excitation, &c., 

 and gives many new facts. 



All solid substances, except the metals, possess the phospho- 

 rescent C[uality. We may, however, by making a judicious selec- 

 tion of the bodies which are to serve as our means of experiment, 

 disembarrass the inquiry of many of its complications. If we 

 employ the sidphuret of barium, or any other substance liable to 

 undergo chemical changes in the air, we introduce unnecessary 

 phsenomena, and cannot distinctly prove whether the shining is 

 due to a cUreet comljustion of their parts, or to other causes. 



Among the selections which might be made, diamond and 

 fluor-spar possess qualities rendering them very eligible for these 

 pmi)Oses, — absolute un changeability in the air and imder water. 

 Even between these there is a choice ; for fluor-spar possesses 

 all the good qualities of diamond, and seems specially adapted 

 for the piirpose. It might be said, considering the chemical 

 relationships of diamond, that when it glows it undergoes a kind 



